The invention relates to a method for forming a monocell or a bi-cell for a lithium-ion electric energy accumulating device.
In particular, the method provides the lamination of the monocell or bi-cell components to make them adhere to one another.
It is known to form a monocell or a bi-cell by laminating the cell forming elements through a pair of rollers. These elements typically comprise at least an anode, a cathode and two separating elements. In the case of the bi-cell, in addition to the listed elements this includes a second cathode as well. The anode, or the cathode, is interposed between the two separating elements. One of the two separating elements is interposed between the anode and the cathode in order for the cell to work properly when in use.
As is well-known to the person skilled in the art, a separating element has a microporous structure in which the micropores are intended to accommodate an electrolyte.
A problem that may arise in the formation of cells relates to the closing of the micropores of the separating element during lamination due to the high compression force exerted on the separating elements by the rollers. This deteriorates the features of the cell and, as a result, worsens the quality of the electric energy accumulating device. On the other hand, should the laminating rollers exert a very small compression force (so as not to close the micropores), there is a risk that such force be not sufficient to guarantee adhesion between separating elements, anode and cathode.
Another problem that may arise in the formation of monocells or bi-cells relates to the geometrical stability of the latter. Indeed, a monocell may deform (in particular, it may bend) due to the high temperatures to which it is subjected during lamination. This is due to the fact that both the anode and the cathode have different thermal expansion coefficients from each other. In other words, when the anode and the cathode are heated to high temperatures (such as during a lamination process), the amounts of expansion are different from each other, whereby this causes cell deformation. A deformed cell, this being substantially curved in cross-section, has several drawbacks. In particular, a deformed cell involves problems relating both to the moving thereof (for example, the cells cannot be stacked on top of each other)) and to the subsequent formation of the electric energy accumulating device, whereby this may have a different shape from the expected one, and thus its shape may not be complementary to the one of the housing arranged to accommodate it.
Still another problem relates to the high temperatures and compression forces occurring during the lamination step, whereby these may damage the electrodes (anode and cathode), in particular the cathode.